Once you have InstallAssistant_highsierra.dmg, follow these steps:
sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
Sometimes the link above triggers a small "stub" downloader. If you need the complete installer for a bootable USB drive, use the Apple Support page. download macos high sierra 1013 6 dmg file link
The App Store download provides an application installer, not a DMG file. To get the DMG file required for bootable media or virtual machines, you must convert the downloaded app. Once you have InstallAssistant_highsierra
Tools Required: A Mac and a USB drive (8GB+) or a blank disk image. Type your password, confirm “Yes” to erase, and
Option A: Using Terminal (CreateInstallMedia) Apple provides a built-in tool to create a bootable installer, which formats a drive with the necessary OS files.
Option B: Creating a DMG for Virtual Machines (VMware/VirtualBox)
If you need a specific .dmg file rather than a bootable USB, you can create one from the downloaded App: